The present invention relates to ultrasonic imaging techniques for obtaining information about tissue elasticity and in particular to a method of rapidly acquiring three-dimensional elasticity reconstructions useful, for example, during RF ablation.
Elastography is an imaging modality that reveals the stiffness properties of tissues, for example axial strain, lateral strain, Poisson's ratio, Young's modulus, or other common stiffness measurements. The stiffness measurements may be output as quantitative values or mapped to a gray or color scale to form a picture over a plane or within a volume.
Generally, stiffness is deduced by monitoring tissue movement under an applied force or deformation. The monitoring may be done by any medical imaging modality including computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonic imaging. Elastography of this type is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount that the tissue yields under pressure.
In “dynamic” elastography, a low frequency vibration is induced in the tissue and the velocity of the resulting compression/shear waves is measured, for example, using ultrasonic Doppler detection. In “quasi-static” elastography, two images of the tissue are obtained at different states of compression, typically using the ultrasonic transducer as a compression paddle. Displacement of the tissue between the two images is used to deduce the stiffness of the tissue.
U.S. Pat. No. 7,166,072, assigned to the same assignee as the present invention and incorporated by reference, describes a novel technique for monitoring a radiofrequency ablation using quasi-static elastography. Radiofrequency or microwave ablation is a process for treating tumors or the like which employs one or more electrodes inserted percutaneously to the site of a tumor. Ionic heating of the tissue induced by radiofrequency fields in the tissue kills tumor cells and produces a hardened lesion. This lesion, being much stiffer than the surrounding tissue, may be monitored by quasi-static elastography using the ablation electrode as the compression device. Adhesion between the ablated tissue and the electrode allows the source of the compression to be at the site of the tumor (as opposed to external compression to the patient) providing a more accurate characterization of the stress field near the tumor and, accordingly, substantially improved elastographic measurement.
The present inventors have also developed a method of evaluating tissue elasticity by monitoring the propagation of shear waves extending generally perpendicularly to an axis of the ultrasound. The shear waves may be induced, for example, by reciprocation of an ablation probe. The speed of the shear wave is dependent on tissue elasticity, and may be extracted from the ultrasound image to reveal information about the size and growth of an ablated region. This process is described in U.S. Pat. No. 8,328,726 issued Dec. 11, 2012, assigned to the assignee of the present invention and hereby incorporated by reference.
Generally, these techniques may be used to produce three-dimensional elasticity data and images, for example, by sliding or rocking the ultrasound transducer to obtain multiple image planes within a volume. The data of these planes may be collected to produce a three-dimensional image. Substantial time is required to acquire the necessary data for these three-dimensional techniques limiting their usefulness for monitoring a real-time process such as RF ablation. Acquiring three-dimensional data sets is particularly time consuming when multiple registered images need to be obtained at each location as is often the case with elastography. Although data volumes can also be acquired directly using 2D ultrasound array transducers, the use of such technology is currently limited due to the high cost of manufacturing such sensor arrays.